Second proposal

Now there is a grey line is difficutly is unknown or extreme (??) and quasi invisible difficulty-colored dots when grooming is unknown. I don't know if this sort of rendered FIXME is of use, but I think this could encourage mappers to improve their mapping.

Third proposal

I try to have various rendering style with an automatic color for difficulties. Downhills went slightly transparent with white underline to differentiate them from others.
Almost there, any idea for ice-skating ?